1 Field of the Invention
This invention relates to a methacrylic resin composition and a method for the production thereof. More particularly, it relates to a methacrylic resin composition useful as a disk substrate material for such optical information memory disks as the video disk and a method for the production thereof. 2. Description of the Prior Art
The optical video disk generally comprises a disk substrate, memory pits having about 1 .mu.m long, about 0.4 .mu.m wide, and about 0.1 .mu.m deep incised in one surface of the disk substrate, an aluminum film vacuum deposited thereon so as to cover the memory pits, and a protective coating formed to cover the aluminum film. The disk substrate is produced from methacrylic resin by an injection molding device having attached to a mold thereof a stamper produced from a master disk and, therefore, is obtained in the form of a replica to which the pits have been transferred from the stamper surface. The information loaded in the pits is reproduced by a procedure which comprises projecting a laser beam on the disk substrate and reading the light reflected on the pit surfaces. The resin to be used for making the disk substrate, therefore, requires the possession particularly of such qualities as (1) excelling in the ability to permit exact transfer of memory pits during the injection molding and allow smooth release of the disk substrate from the mold (hereinafter referred to collectively as "moldability"), ( 2) ensuring a value of smaller optical strain (birefringence), (3) precluding the inclusion of very minute contaminants liable to interrupt the light path, and (4) excluding such chemical impurities as would entail gradual corrosion of the aluminum film among other qualities expected of the resin as a molding material.
In recent years, the growing densification of information to be stored in the disk has been growing and the need to improve the productivity, namely, to shorten the cycle of injection molding has been urging. Since the resin must be cooled at an increased speed as a result, the very minute memory pits in the stamper are required to be exactly transferred to the resin side within the mold which is kept at a lower temperature than usual heretofore. As a result the replica is released from the stamper while it has not been fully cooled and still has a plasticity to some extent. Therefore, the improvement of the "moldability" of resin is important.
In order to improve the "moldability" of the resin it is useful to improve the flowability of resin, that is, to decrease the polymerization degree or to increase the content of a co-monomer of the resin, which invariably resulting in decrease of mechanical strength and heat distortion temperature. Accordingly, in practical use the optimum condition of the resin have been selected delicatelly (JP-A-57-123,208).
The practice of using a mold release agent has been prevailing to date for the purpose of improving the release of a molded resin from the mold. For example,
(1) A method which comprises using a methacrylic resin containing 0.01 to 0.1% of a hydrocarbon represented by the general formula, C.sub.n H.sub.2n+2 (JP-A-58-6,539), PA1 (2) A method which comprises adding 0.1 to 3.0 parts by weight of a surfactant containing a fluoroalkyl group as a lubricant to a methacrylic resin (U.S. Pat. No. 4,510,279), PA1 (3) A method which comprises adding to a methyl methacrylate polymer 0.05 to 2.0 parts by weight of at least one member selected from among fatty acid monohydric alcohols, cetanol, stearic acid glycerol esters, and perfluoroalkyl group-oleophilic group-containing oligomers (U.S. Pat. No. 5,252,440), PA1 (4) A method which comprises adding a glycerol higher fatty acid ester and a saturated aliphatic alcohol having 1 to 30 carbon atoms in a combined amount in the range of 500 to 5,000 ppm to a methyl methacrylate copolymer (JP-A-1-294,763), and PA1 (5) A method which comprises adding 500 to 5,000 ppm of glycerol higher fatty acid monoester and optionally further adding 500 to 3,000 ppm of stearyl alcohol to a methyl methacrylate copolymer (JP-A-4-53,860) have been proposed. PA1 Column: Capillary column G-230, 1.2 mm diam..times.40 m PA1 Column temperature: Kept at 50.degree. C. for 4 minutes and elevated from 50.degree. to 270.degree. C. at a rate of 15.degree. C./min. PA1 Injection temperature: 250.degree. C. PA1 Detector temperature: 270.degree. C. PA1 Cylinder temperature: 270.degree. C. (nozzle side), 250.degree. C. (middle part ), 230.degree. C. (hopper side) PA1 Nozzle temperature: 260.degree. C. PA1 Mold temperature: 50.degree. C. PA1 Injection pressure: 1,000 kg/cm.sup.2 PA1 Injection ratio: 200 co/sec. PA1 Molding cycle: 12 sec. (including a cooling time of 6 sec.)
If the cycle of molding is shortened for the purpose of improving the productivity of the disk, however, the use of any of the methods cited above will not attain expected "moldability", resulting in lowering the yield of production. In the method cited above, the release of the replica (disk substrate) from the stamper is not sufficiently smooth in case of lower content of lubricant. In that case the shape of replicated pit is not so perfect that the disk has some defect of signal, so-called "drop out". On the contrary in case of higher content of lubricant, condensate of volatile gas accumulate on the surface of the stamper, the mold and in the vent zone of the mold during long time operation of injection. In that case the disk has some defilement on the surface resulting in some defect of signal.
An object of this invention, therefore, is to provide a methacrylic resin composition excelling in the "moldability" and causing no defilement on the surface of the mold, and a method for the production thereof.
Another object of this invention is to provide a methacrylic resin composition capable of permitting exact transfer of memory pits and smooth mold release during the continuous formation of disk substrates by injection molding, incapable of causing defilement of the stamper, the mold, and their vicinities, and proving suitable as a molding material for disk substrates of various disks such as the laser disk and a method for the production thereof.